The claimed invention relates to a novel, halide-free, environmentally-friendly (i.e., “green”) method of producing organic poly(disulfide) polymers and networks through, thiol oxidation. Polysufidic polymers are known in the art and are frequently referred to as “thiokols.” Poly(disulfides) typically display high resistance to hydrocarbon solvents and have a low glass transition temperature. One previous method of polysulfide polymer synthesis focuses on the reaction of α,ω-dihalogenated compounds with polysulfide salts. The typical “Thiokol method” of polysulfide synthesis may be shown as follows:
Disulfide and polysulfide bonds of the sulfide salt are created in situ prior to the reaction with the dihalide. The rank of sulfide bonds, from 1 to 5, is controlled by the method of synthesis. Many side reactions are possible in this method of synthesis. Some of the side reactions may be controlled to produce the desired end groups, or to tailor the molecular weight. The products of other side reactions, however, must be removed once the polymer has been formed.
Subsequent research has lead to other methods of preparation. Dithiols have been polymerized to polydisulfide polymers by oxygen alone, but the uncatalyzed reaction was too slow to become a practical industrial process, and high molecular weights were not achieved.
Oxygen- or air-induced oxidation of dithiols was previously known. One type of reaction included the addition of magnesium chloride and sodium hydroxide and often heated the reaction medium to boiling temperatures. Air oxidation has also been used to increase the molecular weight of previously synthesized polymers having terminal thiol groups. Neither molecular weight nor viscosity data were provided in published data. The texture of the resulting polymers was documented. For example, polymers created from the reaction of ethylene dichloride with sodium-sulfur salts were described as brittle, hard plastics, while the polymer formed from linear alkoxy halides were described as liquids or rubbers. Suspended dithiols (1-β-mercaptoethyl-3(or 4)-mercaptocyclohexane, 1-(α-methyl-β-mercaptoethyl-3-mercapto-4-methylcyclohexane, 1,6-hexanedithiol, 1,7-heptanedithiol, 1,9-nonanedithiol and 1,10-decanedithiol) in aqueous solutions of lauric acid and potassium hydroxide have been synthesized by bubbling compressed air through the emulsion for four to ten days. Selenious acid was also added as a catalyst in some polymerization reactions. Several other oxidizing agents (bromine, nitric acid, and ferric chloride) were investigated, but it was previously found that air oxidation produced polymers with the highest viscosities. The inherent viscosity (in chloroform) of the polymers as well as what they called the “softening temperature”, which ranged from 45 to 130° C., were analyzed. The softening temperature may be comparable to what is currently known to as the glass transition in amorphous polymers and melting temperature in semicrystalline polymers.
Using a pure oxygen atmosphere, α,ω-thiol functionalized triethylene glycol and polyethylene glycol oligomers have been oxidized to linear polymers. The monomers and oligomers may be dissolved in an ammonia/methanol solvent mixture and exposed to pure oxygen gas atmosphere while stirring. After a 50 hour reaction period, the polymer from triethylene glycol reached Mn=61,000 g/mol. The polymer formed from the polyethylene glycol oligomer did not exceed ˜40,000 g/mol.
Hydrogen peroxide may be used as an oxidizing agent in many synthetic organic chemistry reactions, including the oxidative polymerization of aniline and phenols and in the oxidation of small molecule thiols to disulfides. In the oxidation of thiols, the reaction does not stop at the disulfide, and may easily be over oxidized to form a sulfonic acid.
For this reason, it has been seen to have limited use as a thiol oxidizing agent. Reaction with hydrogen peroxide can also be a very exothermic reaction, which may make it unattractive for some purposes.
Hydrogen peroxide has been used as the oxidizing agent in the preparation of disulfide oligomers intended to modify Thiokol prepolymers. 2,2′-thiodiethanethiol or 2,2′-oxydiethanethiol was reacted with aqueous hydrogen peroxide in the presence of sodium hydroxide to produce colorless, viscous liquid oligomers of the corresponding dithiol. Molecular weight analysis of the oligomers was not provided in the published disclosure.
Hydrogen peroxide was also documented in dithiol polymerization reactions in other published techniques. However, the dithiol compounds were first reacted with elemental sulfur in the presence of a base. Also, the polymerization reaction was performed in an alcohol solution that contained an inorganic base like sodium hydroxide.
Triethylamine has been used as a catalyst for the air oxidation of thiols to disulfides dissolved in dimethyl formamide, and as a catalyst in the fragmentation polymerization reaction that combines dithiols and diacylsulfides to produce poly(disulfides).
While oxygen, hydgrogen peroxide and triethylamine have been indicated in separate oxidative and oxidative polymerization reactions of dithiols to poly(disulfides), previous art documenting the combination of the three, without other additives, and leading to high molecular weight polymers has not been known. Additionally, other methods either use a chlorine-containing compound or an organic solvent as the reaction medium.
There is a need, therefore, for an alternate method for, synthesis of organic poly(disulfide) polymers. There is also a need for a method of synthesis of organic poly(disulfide) polymers that does not rely upon the use of a heavy metal catalyst or halogenated compounds. There is likewise a need for a method of synthesis of organic poly(disulfide) polymers that does not rely upon the use of an organic reaction media.